In one typical application of the invention, the reactor serves as a reformer to convert reactants such as steam and methanol into a hydrogen-rich fuel usable in electrochemical fuel cells. The reaction process, referred to as reforming or steam reforming, may be represented by the following equation: EQU CH.sub.3 OH+H.sub.2 O+Heat.fwdarw.3H.sub.2 +CO.sub.2
Typical industrial reformers are large, fixed-site units designed for continuous operation at roughly constant throughput. The reactor vessel typically employs a conventional shell-and-tube design in which reactants pass through tubes containing catalyst while heat is applied, usually in the form of hot gases contained within the shell, to the outside of the tube.
Most efforts to date to produce small reformers for mobile applications and other applications requiring a compact design have used modifications of the industrial shell-and-tube concept. For example, U.S. Pat. No. 4,737,161 discloses a reformer for use in reforming a raw hydrocarbon feedstock into a hydrogen gas suitable for use in a mobile fuel cell power plant. The reformer comprises a cylindrical housing, a cylindrical flame tube within the housing, and a helical tube containing catalyst for promoting the reforming reaction. The helical tube is located in the annular space between the flame tube and the reformer housing. Hot gases supply the necessary heat of reaction. A disadvantage of this design is that, while it may result in a more compact reformer than conventional shell-and-tube reformers, it still cannot achieve the compactness of a plate-and-frame design reformer. Moreover, it does not exhibit the modularity of the present design. For example, the capacity of the present plate-and-frame design can be enlarged by simply adding more plate-and-frame units to the reactor stack.
Plate-and-frame type reformers are known in the prior art. For example, U.S. Pat. No. 4,933,242 discloses a power generation system which includes a fuel cell section comprising a cell stack including alternately interleaved fuel cell elements and separator plates. The power generation system also includes a plate-type reformer assembly with gas passages extending within the cell stack and the reformer assembly. A disadvantage of this design is that the reactants pass through the plates at a relatively lower linear velocity than that achieved in the present design. A lower linear velocity results in a thicker boundary layer along the reactor walls, which inhibits the efficient transfer of heat from the heated side of the plates to the reactor side.
The present invention offers a number of advantages over prior art structures. First, the plate-and-frame design coupled with the labyrithine configuration of the reactant flow passage increases the transfer of heat from the thermal fluid to the reactants by providing a high ratio of heat transfer surface area to reactor volume and by increasing the linear velocity of the reactants through the reactor for a given space velocity. The high ratio of heat transfer surface area to catalyst bed volume provides more surface area for heat transfer to occur. The high linear velocity of the reactants through the reactant flow passage reduces the thickness of the boundary layer, which improves the transfer of heat from the walls of the reactant flow passage to the reactants. The use of extended heat transfer surfaces also increases the turbulence of the reactant gases, resulting in a higher conversion of feedstock to reformate fuel.
Second, the plate-and-frame design provides a more compact overall unit than the conventional shell-and-tube design. For example, the heat exchange section of the present invention requires only about one-fifth the volume and weight of a conventional shell-and-tube heat exchanger.
Third, the plate-and-frame design results in a high degree of modularity. In particular, if more reformer capacity is needed, additional plate and frame units may be connected in a stacked arrangement.
Finally, the use of a pumpable thermal fluid instead of combustion gas to provide heat to the reactor improves temperature control. Because of the high heat capacity of the thermal fluid, more heat can be provided to the reactor for a given period of time. Due to the high rate of heat transfer and the variable thermal mass inherent in the thermal fluid, start-up times are faster. Also, the use of a thermal fluid provides favorable load following characteristics because of the high heat capacity of the thermal fluid. The use of a pumpable thermal fluid which is heated in an external reservoir creates a significant thermal capacitance in the reservoir which allows for changes in heat delivery required by changes in fuel cell load.